Recently, handheld electronic devices such as mobile phone and the like have been improved for higher performance and smaller size such that higher energy density and smaller size of batteries used in the handheld electronic devices are desired. In general, a lithium battery can provide a high voltage and achieve a high energy density so as to be expected to be utilized as the power source for such handheld electronic devices. In such lithium battery, lithium transition metal complex oxide such as lithium cobaltate (LiCoO2), lithium manganate (LiMn2O4), lithium nickelate (LiNiO2), etc., is generally used as a positive electrode active material. As a negative electrode active material, a carbon material such as graphite, fibrous carbon, and so on is used. An organic electrolyte solution is generally used in such lithium battery, and a polymer electrolyte, in which a macromolecular electrolyte and an organic electrolyte solution are mixed, is also being investigated. Since a liquid electrolyte is used in such lithium battery or polymer electrolyte battery, leakage and ignition of the liquid electrolyte can be caused such that the reliability of the battery is low. Also, since the battery performance may be drastically lowered if electrolyte solution freezes at a low temperature or vapors at a high temperature, an operating temperature range of the battery is limited. Therefore, the research and development of the lithium battery as a highly reliable battery using a solid electrolyte having a lithium ion conductive property instead of the organic electrolyte solution is being desired.
Since such a solid state battery does not use a flammable organic solvent, it is free from leakage of the solvent and fire therefrom such that excellent safety is provided. For example, Japanese patent application publication No. 2007-227362 discloses a method of manufacturing a laminate by forming an active material green sheet and a solid electrolyte green sheet, respectively; laminating the solid electrolyte green sheet on one face of the active material green sheet; forming a current collector green sheet layer on the other face of the active material green sheet; heating thus-laminated body at a temperature from 200 to 400° C. in an oxidizing atmosphere; and sintering at a higher sintering temperature (for example, from 700 to 1000° C.) in a low oxygen atmosphere, Therefore, even if the current collector made of metal material is oxidized during the heating in the oxidizing atmosphere, the oxidized current collector can be reduced during the sintering at the higher sintering temperature (for example, 700 to 1000° C.) in the low oxygen atmosphere. Although addition of glass frits to a current collector slurry is referred to in Japanese patent application publication No. 2007-227362 document, no technical effects thereof are described.
Meanwhile, Japanese patent application publication No. 2007-5279 discloses a laminate comprising: an active material layer; and a solid electrolyte layer bonded to the active material layer by sintering. The active material layer contains a crystalline first substance capable of adsorbing and desorbing lithium ions and the solid electrolyte layer contains a crystalline second substance having a lithium ion conducting property. Here, at least one of the active material layer and the solid electrolyte layer contains an amorphous oxide. The amorphous oxide, for example, may comprise SiO2, Al2O3, Na2O, MgO, CaO, etc. Japanese patent application publication No. 2007-5279 states such amorphous oxides are added as a sintering additive such that temperatures at which sintering starts and sintering rates in various kinds of materials can be conformed to the common ones although they may differ depending on the kinds of materials.